Evolution - How did different systems of organs come into being?

Different organs evolve from modification or additions to existing organs and body plans. Take, for example, the origin of the bird's crop. The crop is found in some birds only, not in all birds. It is a pouch that holds additional food and is located above the stomach. It is clear that a crop is useful for birds that cannot digest their food fast enough, but rather useless if a bird eats a big meal and does not need more food until after it has digested its big meal. The crop is formed by enlarging the esophagus, and it is not a complicated structure to evolve. Neither is the stomach. It is just an enlarged part of the gut. However, the stomach is more than that because it does secrete acid to digest proteins. Even simple animals like jellyfish have a primitive stomach, in which food is digested. Jellyfish only have one single opening into the body cavity, and both food and waste pass through this opening. More complicated animals evolved a second opening, and it has happened more than once, because in one group of animals, the first opening that is formed in the embryo becomes the anus, but in a second group of animals that opening becomes the mouth.

If the digestive system was designed by some intelligent supernatural being, then there is no reason why a designer should choose to use two different ways to create a mouth and an anus, and also no reason why the designer should give some animals only a single opening for both food and feces but give other animals different openings for food, urine, feces, and babies (e.g. human females) but the same opening for both sperm and urine in the male human. It is these inconsistencies which the creationist cannot explain, but which are perfectly consistent with evolutionary tinkering with pre-existing structures, which have convinced most people that evolution is fact.